4.6 L (W) 1998 Expedition, Head Gasket
 

What is the average amount to pay to have the head gaskets replaced. I have some light colored oil on the bottom portion of the oil cap, milkshake looking. I am not sure if that stuff in the bottle (head Gasket sealer) works. On the dipstick itself, you cant see anything but the normal color and it doesnt smoke, run poor. Thanks for any help/advice.

You just need to drive the thing for awhile. Moisture builds up in the cap from short trips. The engine doesn't run long enough for the oil to get good and hot and boil out any moisture in the crankcase. I suggest at least once a month taking a nice long (50 mile or so) drive to prevent this from happening. Your engine is fine.